Remove references to openjdk6

• Remove references to "6" in comments and examples
• Remove java/openjdk6 from the list of Java ports that are considered to satisfy a dependency
• Don't yet remove "6" as a valid version since many ports have "1.6+" as a version specifier (although none have just 1.6). Instead force this to mean "1.7+".

PR: 234792
Sponsored by: The FreeBSD Foundation